Plain English Meaning
A ground site at a known, exact location that listens to GPS satellites, works out how far off the GPS signals are, and sends that information up the chain so aircraft can receive corrected, more accurate GPS positions.
Full Definition
A precisely surveyed ground station, part of the Wide Area Augmentation System (WAAS), that continuously receives GPS satellite signals and measures the errors in those signals. The station forwards its measurements to a WAAS master station, which builds correction data that is then broadcast to aircraft via geostationary satellites to improve GPS accuracy and integrity.